This invention relates to a magnetic sensor exhibiting variation of an impedance in dependence upon the strength of an external magnetic field and, in particular, to a magnetic sensor for use as a magnetic head and the like.
In recent years, rapid progress has been made in the development of small-sized and high-performance electronic apparatuses. As a reading head for small-sized and large-capacity hard disks in computer-associated apparatuses, a conventional magnetic head utilizing electromagnetic induction is being replaced by a magnetoresistive head (MR head) utilizing a magnetoresistance effect. However, even the MR head is insufficient to meet further increases in recording density. Under the circumstances, there is a strong demand for a new magnetic element exhibiting wide variation in electric characteristics in response to variation in strength of an external magnetic field.
Besides, the MR head is insufficient for use in measurement and detection of a weak magnetic field, such as measurement of geomagnetic strength or the magnetic field of a brain.
In view of the above, proposal has been made of a magnetic sensor (also referred to as a "magnetic impedance element") comprising a soft magnetic wire to be supplied with a high-frequency current. In response to the variation in strength of the external magnetic field, the soft magnetic wire exhibits a variation in resistance and inductance, namely, a variation in impedance. Thus, the impedance variation is utilized in detection of magnetic field strength (Japanese Unexamined Patent Publications Nos. 176930/1994 and 248365/1995, Proc. of The Institute of Electrical Engineers of Japan, Vol. E116, No. 1, page 7 (1996)). Such magnetic sensor exhibits wide variation in impedance in response to the variation in strength of the external magnetic field and therefore has a good performance for a magnetic head. However, the impedance variation rate (i.e., magnetic field sensitivity) in dependence upon the variation in magnetic field strength is as small as 10%/Oe.
In order to remove the above-mentioned disadvantage, proposal has been made of another magnetic sensor comprising an oscillation circuit formed by a combination of a transistor and a soft magnetic wire. With this structure, LC resonance is utilized to improve sensitivity (Journal of The Magnetics Society of Japan, Vol. 19, page 469 (1995)). However, this magnetic sensor not only requires active components but also a plurality of resistors, capacitors, and diodes. Therefore, the production cost inevitably becomes high.
On the other hand, consideration is made about the use of a single layer of an amorphous metal magnetic film in order to realize a small-sized magnetic sensor (Proc. of The Institute of Electrical Engineers of Japan, 115-A, page 949 (1995)). In this magnetic sensor, an electric current is directly supplied to the magnetic film so as to detect impedance variation in dependence upon the external magnetic field. However, as compared with those metals, such as Cu, Al, and Ag, generally used as a conductor line, the amorphous metal magnetic film has a large electrical resistance. Accordingly, efficient excitation can not be carried out and the impedance variation rate is small.
Furthermore, proposal has been made of a yet another magnetic sensor or magnetic impedance element comprising a sputtered permalloy film of a stripe pattern including a Cu film (Senda et al, The Institute of Electrical Engineers, Technical Meeting on Magnetics, MAG-95-126, 91 (1995)). In addition, proposal has been made of a different magnetic sensor comprising CoSiB films with uniaxial magnetic anistropy introduced and a Cu conductor layer interposed therebetween (Morikawa et al, Journal of The Magnetics Society of Japan, No. 20, page 553 (1996)). These magnetic sensors exhibit an impedance variation rate between -50 and +120% with respect to the external magnetic field varying within a certain range. However, magnetic field sensitivity is no more than -5 to +10%/Oe. In addition, it is difficult to control the magnetic anisotropy.
In the meanwhile, in the above-mentioned magnetic sensors using soft magnetic elements, the impedance of the magnetic sensor is increased in a frequency band on the order of several tens to several hundreds megahertz (MHz) under the influence of the skin effect and the increase in eddy current loss. This means that the impedance variation in response to the variation in strength of the external magnetic field is relatively small.
In the above-mentioned conventional magnetic sensors, stray capacitance is produced between the magnetic sensor and other circuit components or conductor lines present around the magnetic sensor. This results in unstable operation of the magnetic sensor.